1. Field of the Invention
The invention is directed to molding compositions comprising selectively hydrogenated styrenic block copolymers and polypropylene or polypropylene random copolymers, and additional clarifying agent. The invention is also directed to molded articles made from the molding compositions. The molding compositions and the articles made from them have a unique combination of low temperature impact strength and low haze (good clarity). Although such blends have unique balance of good impact strength and low haze at low temperature, the same unique properties of improved impact strength and low haze are also present at room temperature (20 to 30° C., for example).
2. Prior Art
It is known that polypropylene or polypropylene random copolymers can make good molded articles such as food storage containers, for example. Polypropylene and polypropylene random copolymers, however, tend to crystallize with large crystallites and the haze of such containers is often quite high. Random polypropylene copolymers tend to be slightly clearer than the polypropylene homopolymer and are thus usually preferred for ultimate clarity. It is known that adding a small amount (0.18 to 0.22 wt. %) of a clarifying agent causes the crystallites of the homo-polypropylene or polypropylene random copolymer to remain small and become more numerous, thus reducing the haze significantly. Although such food containers made from polypropylene or polypropylene copolymers with clarifying agents have low haze, they have poor toughness and impact strength, especially at low temperatures. Specifically, such containers often shatter because they become very brittle at temperatures of 0° C. or lower. In order to improve the impact strength of polypropylene molded articles, it is known to add elastomeric polymers.
U.S. Pat. No. 4,904,731 issued to Holden and Hansen (and assigned to one of the assignees of the present invention) discloses a polymeric composition comprising homo-polypropylene and polypropylene random copolymers, a hydrogenated block copolymer containing predominantly monoalkenyl aromatic hydrocarbon monomer units and at least one block containing predominantly hydrogenated, conjugated diolefin monomer units and linear low density polyethylene. The hydrogenated styrene block copolymer used in the examples had 30 wt % styrene and the best ethylene copolymers had a density around 0.917 g/cm3. Such a polymeric composition is stated to have or exhibit improved clarity and good impact strength when mixed together in a ratio of 1:1. It is noted, however, that this composition was only tested for Gardner impact strength at −10° C. at levels of 12.5-15 wt % impact modifier (the hydrogenated styrene block copolymer). Moreover, the clarity was not as good as achieved in this present invention, nor is there is any mention of the clarifying agents or the criticality of the level of clarifying agent which according to this present invention is key in achieving the best clarity. Lastly, the hydrogenated styrene block copolymer and the polyethylene copolymer used in the examples were not optimum for achieving the best match of the index of refraction with the polypropylene random copolymer which is important to achieve the best clarity. There was also no mention for the need to match the index of refraction which is important to achieve good clarity even if high levels of clarifying agent are used.
Polypropylene polymers can achieve low temperature impact at −20° C. to −30° C. if high levels of ethylene copolymers with low densities (typically 0.86 to 0.87 g/cc) are added or if the polyethylene copolymers are polymerized in situ (typically referred to as polypropylene impact copolymers) but these copolymers have extremely poor clarity (90-100% haze in 0.125 inch thick injection molded articles) because of the poor index of refraction match and large rubber particle size. Thus there is a need in the art to create blends of homo-polypropylene or polypropylene random copolymers with an elastomeric polymer that would provide ample clarity (a haze level of less than about 50% haze for a 0.125 inch thick injection molded disk or article), while maintaining sufficient impact strength to prevent a molded article, such as a food container for freezers, from cracking or shattering, especially at −20° C. to −30° C.